I have a lifted 03 H2. The rear sags on the left side and on the front driver side. I think it is the rear coil spring. Any other ideas?

 

Mine did same thing...

It's due to several things:
-gas tank on left
-driver on left
-steering column on left
-front right wheel is drive-wheel so it throws weight to rear-left when accelerating

Couple of things to address it:
-new coil springs
-add airlift air bags with seperate valves for independant control
-swap coil springs

I did the air bags and run 30#s psion the left side and 15#s psi on the right. The cost was only $90 and now when I take off from a dead stop and is so much more steady!

 

One other note...keep in mind that the factory shocks were 300psi each so they provided some "spring rate assistance" to the rear coils. Most likely, you changed these out to match the lift kit and this would have speeded up the process of wearing out your left coil spring.

 

You may also want to check your torsion bars.....and adjust the keys. Depending on how many miles you have, you may have a torsion bar on the driver's side that is a little more worn out, and will need to be adjusted up.